The present invention relates to a belt driving device for a copier, printer, facsimile apparatus or similar image forming apparatus and an image forming apparatus using the same.
One of conventional image forming apparatuses of the type using a belt driving device includes an endless belt facing image carriers. The belt conveys a paper sheet or similar recording medium, so that toner images are transferred from the image carriers to the paper sheet one above the other. The image carriers are often implemented as photoconductive belts or intermediate image transfer belts. While such belts each are passed over a plurality of rollers, it is apt to deviate in the direction perpendicular to the direction of movement (axial direction of the rollers). In the worst case, the belt slips out of the rollers. The deviation of the belt does not occur if the belt moves in an ideal condition in which the parallelism of the rollers and the thickness, circumferential length and tension of the belt are free from errors and irregularity. In practice, however, the errors and irregularity are not avoidable unless the accuracy of the individual part and accurate assembly are enhanced, resulting in an increase in cost. It is therefore necessary to use a mechanism for preventing the belt from deviating.
In light of the above, it has been proposed to position a guide member for limiting the deviation on the inner surface of the belt at one of opposite edges in the direction of width, and form an annular groove in each roller for receiving the guide member. This configuration obviates the deviation of the belt at lower cost than a configuration in which guide members are provided on both edges of the inner surface of the belt.
However, the above prior art scheme using a single guide member has the following problems left unsolved. When the belt in movement deviates to the side where the guide member is positioned, the image transfer surface of the belt slackens or creases and brings about defective image transfer. If the belt with the slackened or creased image transfer surface further moves, then the guide member is apt to slip out of the grooves of the rollers and get on the rollers.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication
It is an object of the present invention to provide a low cost, belt driving device capable of causing a belt to stably move without slackening or creasing and without a guide member getting over the guide portions of rollers, and an image forming apparatus using the same.
In accordance with the present invention, a belt driving device includes an endless belt member passed over a plurality of roller members. A driving device causes at least one of the roller members to rotate. A biasing device presses opposite ends of at least one of the roller members to thereby apply tension to the belt member. A guide member is formed on the inner surface of the belt member at one of opposite sides in the direction of width of the belt member for guiding the belt member. A guide portion is formed in each roller member and engaged with the guide member. The biasing device is configured such that a biasing force F2 acting on one end of the roller member remote from the guide member of the belt member is greater than a biasing force F1 acting on the other side close to the guide member.
Also, in accordance with the present invention, an image forming apparatus includes a plurality of image stations each including a respective image carrier on which a toner image is formed. A belt driving device drives an image transfer belt that conveys a recording medium to which the toner image is transferred. The image transfer belt conveys the recording medium via image transfer positions, each of which is assigned to a particular image carrier included in each image station, so that toner images are sequentially transferred from the image carriers to the recording medium one above the other. The belt driving device includes an endless belt member passed over a plurality of roller members. A driving device causes at least one of the roller members to rotate. A biasing device presses opposite ends of at least one of the roller members to thereby apply tension to the belt member. A guide member is formed on the inner surface of the belt member at one of opposite sides in the direction of width of the belt member for guiding the belt member. A guide portion is formed in each roller member and engaged with the guide member. The biasing device is configured such that a biasing force F2 acting on one end of the roller member remote from the guide member of the belt member is greater than a biasing force F1 acting on the other side close to the guide member.